1. Field of the Invention
The present invention generally relates to a wheel speed sensor used for vehicles, and more particularly to a wheel speed sensor adapted to generate signals corresponding to the rotating speed of a vehicle wheel.
2. Description of the Related Art
Generally, wheel speed sensors are used for measuring the rotating speed of a driving shaft of a driving wheel. In the conventional wheel speed sensors, one pulse signal is generated for each single rotation of the driving shaft, and it is sufficient to roughly measure vehicle speed when the vehicle is driven under substantially constant driving conditions.
However, when wheel sensors are provided for measuring rotating speed of wheels in order to use the measured rotating speed for improving driving performance and safety of vehicles, a more accurate measurement of rotating speed of wheels is required.
Rotating speed of wheels of an automobile varies from wheel to wheel due to variation of slip ratio. For example, if an excessive driving torque is applied to the driving wheels, the rotating speed of the driving wheels is higher than that of wheels other than the driving wheels. Additionally, if one of the wheels is locked due to an excessive braking force, the rotating speed of that particular wheel is lower than that of the other wheels.
If the difference in rotating speed among the wheels is measured, various types of vehicle driving condition information can be obtained so as to use the information for controlling the operation of the vehicle. Therefore, in order to measure the difference in rotating speed among the wheels, it is required to measure the rotating speed of wheels with high accuracy.
Recently, many wheel speed sensors have been developed. Those wheel speed sensors use basically a rotatable member rotating together with a wheel and a measuring unit for measuring the rotation speed of the rotatable member. One example of such wheel speed sensors utilizes a magnet and a Hall element fixed to the rotatable member. In the example, the rotatable member is formed as a disk, and a plurality of magnets are circumferentially provided on a side surface of the disk apart a predetermined distance from each other. The Hall element is provided in a predetermined position on a stationary member so as to detect magnetic flux crossing the Hall element.
However, the above-mentioned example of the wheel speed sensor has a problem in that positioning of the magnets and the Hall element requires extremely high accuracy. That is, the radial distance from the center of rotation to the Hall element must accurately correspond to the radial distance from the center of rotation to each of the magnets. Additionally, in order to maintain an appropriate intensity of magnetic flux applied to the Hall element, the gap between the Hall element and each of the magnets must be strictly controlled, the gap being formed in the axial direction of the rotation. Accordingly, in the above-mentioned example, a high accuracy is required in part dimensions as well as assembly of parts.
The Japanese Utility-Model Publication No. 55-17173 discloses a wheel speed sensor in which a part of the above-mentioned problems is eliminated. FIG. 1 shows a structure of the wheel speed sensor described in the Japanese Utility-Model Publication No. 55-17173. As shown in FIG. 1, the wheel speed sensor 1 comprises a housing 2, a fixed shaft 3, a detecting coil 4, a magnetic member 5, a fixed tooth member 6, a magnet 7 and a rotatable tooth member 8. The housing made of non-magnetic material is mounted on an outer surface of the fixed shaft 3. The detecting coil 4 is formed of an annular shape, and is situated inside the housing 2. The magnet 7 is also situated inside the housing 2 between the detecting coil 4 and a wall of the housing 2. One end of the magnet 7 is in contact with the magnetic member 5, and the other end is in contact with the fixed tooth member 6. At one end of the fixed tooth member 6, there are formed fixed teeth 6a facing rotatable teeth 8a formed on the rotatable tooth member 8. The rotatable tooth member 8 rotates together with a wheel (not shown in the figure) relative to the fixed shaft 3.
In the above-mentioned structure of the wheel speed sensor 1, magnetic flux generated by the magnet 7 forms a loop surrounding the detecting coil 4 as shown by dashed lines in the figure. That is, the magnetic flux passes through gaps formed between the fixed teeth 6a and the rotatable teeth 8a. As the rotatable teeth 8a rotate due to the rotation of the wheel, the lengths of the gaps between the fixed teeth 6a and the rotatable teeth 8a vary, resulting in variation of magnetic reluctance. Accordingly, the detecting coil 4 of the wheel speed sensor 1 generates a varying voltage signal due to the variation of the magnetic reluctance corresponding to the rotation speed of the wheel.
The wheel speed sensor 1 requires accuracy only in forming the length of the gap between the fixed teeth 6a and the rotatable teeth 8a because deflection of the position of the both teeth hardly effects the magnetic reluctance. That is, the wheel speed sensor 1 requires only that the radial dimension of the fixed teeth member 6 and the rotatable tooth member 8 be accurately controlled.
However, in the wheel speed sensor 1, since the rotatable tooth member 8 must slidably rotate relative to the fixed shaft 3, the rotatable tooth member 8 and the fixed shaft 3 must be formed as separate parts. Accordingly, a small gap is inherently formed between the inner diameter of the rotatable teeth member 8 and the outer diameter of the fixed shaft 3. Since the length of the gap formed at either end of the diameter of the fixed shaft 3 varies as the rotatable tooth member 8 rotates, the magnetic reluctance. This fluctuation generates noise in the output signal from the detecting coil 4. Therefore, there is a problem in that the noise due to the variation of the length of the gap may cause an error in measuring the rotating speed of the rotatable teeth member 8.